Improved handling of labels in letters

Add all labels in the selected language of the letter to the XML file (same structure as code-tables API). So in the XSLT stylesheet it can be referenced with XPATH and not with @@substituion.

In this way, the power of XPATH and XSLT can also be used for labels.

Example: QueryToPatronLetter (we only ever use line1): now each Patron Query Type must have a block with the same content. With labels in the data XML, it can be a block where the correct type is selected with XSLT and XPATH.

It also makes it easier to develop letters outside of ALMA (e.g. VS Code).